Hydrogenated styrenic block copolymers, such as those having a saturated ethylene-butene-1 mid-block (i.e., SEBS), possess good thermal stability. However, use of such copolymers in certain processes, including thermoforming operations, is limited because of poor processability. This is thought to be a result of the high incompatibility of the styrene end-block and the EB mid-block even in the melt state.
Polypropylene (PP) and high levels of process oils have been added to SEBSs in an attempt to improve processability. In the injection molding or extrusion molding of useful parts, the PP/SEBS/oil mixture will form two co-continuous phases, a first continuous phase of PP and a second continuous phase of oil and SEBS. The continuous, high melting-point PP phase enhances the solvent resistance and heat resistance of the compounds. The combination of phases facilitates the production of soft, flexible parts such as wire and cable insulation and automotive interior outer layers such as instrument panels, seats, and the like.
However, difficulties exist in using high levels of process oil in the blends to achieve lower values of hardness because the oil tends to bloom to the surface (“surface blooming”) and/or emit from the polymers during service. Oils also have a yellowish appearance which detracts from the optical properties in the final product. Further, oils tend to emit a distinct odor which detracts from its use in closed or contained environments, such as automotive interiors.
Typically, mineral oils or synthetic oils are added as the processing oil to improve processability. Mineral oils are any petroleum-based oil that is derived from petroleum crude oil and subjected to refining steps, such as distillation, solvent processing, hydroprocessing, and/or dewaxing to achieve the final oil. This also includes petroleum-based oils that are extensively purified and/or modified through severe processing treatments. Examples of commercially available mineral oils include but are not limited to Drakeol from Penreco (USA), Paralux from Chevron (USA), Sunpar from Sunoco (USA), Plastol and Flexon from ExxonMobil (USA), Shellflex from Royal Dutch Shell (UK/Netherlands), and Diana from Idemitsu (Japan).
Other block copolymer blends have been proposed to improve processability and maintain thermal stability. For example, U.S. Pat. No. 4,904,731 teaches polymeric compositions of a C2-C10 olefin polymer, a hydrogenated block copolymer, and an LLDPE, useful for shaped structures having good clarity and good impact strength. U.S. Pat. No. 5,925,707 discloses oil gel compositions of styrene block copolymers, oil, and optionally a polyolefin wax and/or liquid extender. WO 02/31044 discloses a composition of SEBS, polypropylene, and a polydecene having a molecular weight of from about 400 and 1000 g/mol to make flexible products. Other references include U.S. Pat. No. 4,132,698; U.S. Pat. No. 4,960,820; WO 01/18109; WO 2004/014998; and EP 0300689.